Activity which under 0.6.2 displayed as "yesterday" / "2 
days ago" under 0.6.3 displays as "tomorrow" / "in 2 
days".

Sorting by activity has the same ordering as before -- 
only the display seems affected.

I suspect Date.__sub__: the sign applied to the resulting 
Interval when two dates are subtracted appears to have 
changed between the releases.

But date.py feels like deep magic to me -- as a 
temporary workaround I inverted the subtraction in 
templating.DateHTMLProperty.reldate, which gets my 
tracker displaying correctly again.

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Yes, it did change, though I'm not sure why.

From a mail to roundup-devel a while ago:
--
Johannes: The date interval code seems to broken. When
running the demo, the activity column is always in the
future ('in a moment', 'in three hours'). The tests run fine
though.

Richard: That'll be more the fault of the pretty() function.
I suspect that it mis-interprets the sign of the interval
after Anthony's changes which reversed the sign result from
date operations.
--

Assuming there was a good reason for reversing the sign,
your inversion of the subtraction seems like the correct fix
to me.

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Ah, yes, I've found the thread that explains this:
http://sourceforge.net/mailarchive/forum.php?thread_id=3388727&forum_id=1100.
I inverted the subtraction in
templateing.DateHTMLProperty.reldate on both HEAD and maint-0-6.